Requested action not taken: mailbox unavailable Failure sending mail. Try again later

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Hi fellas,
I am sending a file via SMTP activity. Problem here that it’s not consistant, sometimes send works fine, sometimes I get the error above. I tried two SMTP services, same outcome. Any ideas how make it work at all times? Keep in mind I cannot use Outlook activities.

Source: Send SMTP Mail Message

Message: Requested action not taken: mailbox unavailable
Failure sending mail. Try again later

Exception Type: SmtpCommandException

MailKit.Net.Smtp.SmtpCommandException: Requested action not taken: mailbox unavailable
Failure sending mail. Try again later
at UiPath.Mail.Activities.SendMailActivity.EndExecute(AsyncCodeActivityContext context, IAsyncResult result)
at System.Activities.AsyncCodeActivity.System.Activities.IAsyncCodeActivity.FinishExecution(AsyncCodeActivityContext context, IAsyncResult result)
at System.Activities.AsyncCodeActivity.CompleteAsyncCodeActivityData.CompleteAsyncCodeActivityWorkItem.Execute(ActivityExecutor executor, BookmarkManager bookmarkManager)

Best regards,
Anatoly